1. Field of the Invention
This invention relates to a pig launching system and a pig receiving system for use in cleaning subsea flowline. This invention further relates to a method of propelling a pig through a flowline using a pressurized accumulator, a method of assembling a subsea pig receiver in a flowline using a remotely operated vehicle, and a method of collecting a sample from a subsea flowline, using an accumulator connected to a pig launcher.
2. Description of the Prior Art
Conventional surface launched pigging systems for subsea flow lines used in the transport of oil and/or gas, requires two flow lines from the host facility to the well. This doubles the flowline costs as compared with installing a single flowline. For distances where the flowline length exceeds thirty (30) miles, the extra cost associated with installing a second flowline can be highly detrimental to the economic feasibility of the project.
Prior art methods of propelling a pig through a subsea flowline have not used pressurized subsea accumulators. The present invention provides the ability to launch a pig into a single subsea flowline using a pressurized accumulator and to later retrieve the pig in a pig receiver, installed in the flowline.
One aspect of the present invention is directed toward a pig receiving system which comprises a flowline comprising a connector hub mounted on the flowline and a pig receiving inlet. The system further comprises a pig receiver mounted to the connector hub. The receiver comprises an outer body defining an inner volume sufficiently large to contain a pig sized to abrasively clean the flowline. The receiver is capable of receiving a pig launched from a host facility. The receiver further comprises an inlet section adjacent to the inner volume. The outer body of the receiver comprises openings large enough to allow debris to pass through them and small enough to prevent a pig from passing through them. The system of the present invention further comprises a pig retention mechanism mounted in the receiver.
Another aspect of the present invention is directed toward a method of injecting a pig into a flowline comprising placing the pig in a pig launcher housing connected to a flowline. The housing has an inlet and an outlet. The method further comprises opening a flow control device in a connecting line installed between an outlet of a pressurized accumulator and the inlet of the launcher housing for a sufficient amount of time to allow pressurized fluid to flow from the accumulator, through the connecting line, and into the launcher housing to propel the pig into the flowline.
The invention is also directed to a pig launching system in which a pig launcher is mounted to the same connector hub, to which a pig receiver may be connected. The pig launcher is capable of launching the pig into the flowline just past the well tie-in point. From the well tie-in point a valve in the flowline may be opened to allow well flow and pressure to propel the pig to the host facility.
Another aspect of the present invention is directed to a method of assembling a subsea pig receiver in a flowline, comprising attaching a pig receiver having an inlet section to a downline with a sling and lowering the receiver into the sea to a location above a connector hub on a flowline. The method further comprises grasping the receiver with a remotely operated vehicle, using the remotely operated vehicle to place the inlet section of the receiver on the connector hub, and connecting the receiver to the flowline at the interface between the inlet section and the connector hub.
The invention is further directed toward a method of collecting a sample from a subsea flowline. This method comprises a establishing a pressure in a sample collecting vessel that is lower than a pressure in a subsea pressurized flowline to which the vessel is connected. This method further comprises opening a flow control device in a connecting line installed between an outlet of the sample collecting vessel in the inlet of a pig launcher housing in fluid communication with the pressurized flowline, wherein the flowline contains process fluid. The flow control device is opened for a sufficient amount of time to allow process fluid to flow from the flowline, through the launcher, and into the vessel.